1992 Mercedes Benz 400se V8 Two Wheel Drive Automatic 100000 miles

I have a car thats being traded for and the speeddometer and or the odometer is inop where are the cables for these connected and what should I be on the look for to repair this or troubleshoot it myself. The person has told me that the wheels have been changed from standard to 20 inch thats as much as i know about it as I have only verbally communicated with the current owner. The car is being puchased as is. The milage is a pure guess.
Thanks for any insight you can give
Dave